The present invention relates to an article comprising a substrate containing silicon and a barrier layer which functions as a protective environmental/thermal barrier coating and, more particularly, a barrier layer which inhibits the formation of gaseous species of Si, particularly Si(OH)x when the article is exposed to a high temperature, aqueous (water and/or steam) environment.
Ceramic materials containing silicon and metal alloys containing silicon have been proposed for structures used in high temperature applications as, for example, gas turbine engines, heat exchangers, internal combustion engines, and the like. A particular useful application for these materials is for use in gas turbine engines which operate at high temperatures in combustion gas environments. It has been found that these silicon containing substrates can recede and lose mass as a result of a formation volatile Si species, particularly Si(OH)x and SiO when exposed to high temperature, combustion environments having water vapor as a product of combustion. For example, silicon carbide when exposed to a lean fuel combustion environment of approximately 1 ATM pressure of water vapor at 1200xc2x0 C. will exhibit weight loss and recession at a rate of approximately 6 mils per 1000 hrs. It is believed that the process involves oxidation of the silicon carbide to form silica on the surface of the silicon carbide followed by reaction of the silica with steam to form volatile species of silicon such as Si(OH)x. U.S patent application Ser. No. 09/292,349 filed Apr. 15, 1999, now U.S. Pat. No. 6,410,148 discloses a barrier layer for a silicon containing substrate which is effective in inhibiting the formation of volatile silicon species and thereby reduce recession and mass loss in the substrate. The barrier layers disclosed as useful are alkaline earth aluminum silicates. Particularly useful barrier layers include barium alumino silicates and barium strontium alumino silicates. While these barrier layers have been found to be particularly effective on silicon carbide substrates, it is expected that the barrier layers when applied to substrates containing silicon nitride may lead to cracking due to the differences in the coefficient of thermal expansion between the alkaline earth alumino silicates and silicon nitride.
Naturally it would be highly desirable to provide a external barrier coating for silicon containing substrates, including silicon nitrides which would inhibit the formation of volatile silicon species, Si(OH)x and SiO, and thereby reduce. recession and mass loss.
Accordingly, it is the principle object of the present invention to provide an article comprising a silicon containing substrate with a barrier layer which inhibits the formation of gaseous species of Si, particularly Si(OH)x, when the article is exposed to a high temperature, aqueous environment, such as a gas turbine engine hot section.
A second objective of this invention is to provide an article comprising a substrate with a barrier layer providing thermal protection, such layer closely matching the thermal expansion of the substrate.
It is a further object of the present invention to provide a method for producing an article as aforesaid.
The present invention relates to an article comprising a silicon containing substrate having a barrier layer on the substrate, wherein the barrier layer functions to both inhibit the formation of undesirable gaseous species of silicon when the article is exposed to a high temperature, aqueous environment and to provide thermal protection. The term xe2x80x9chigh temperaturexe2x80x9d refers to the temperature range at which the Si in the substrate forms Si(OH)x and/or SiO in an aqueous environment. The term xe2x80x9caqueous environmentxe2x80x9d refers to a water and/or steam environment. The silicon containing composite is preferably a ceramic or metal alloy containing silicon including, but not limited to, silicon nitride, silicon carbide and Moxe2x80x94Si alloys. The barrier layers in accordance with the present invention comprises an alkaline earth metal aluminosilicate and an oxide selected from tantalum oxide, niobium oxide and mixtures thereof. The addition of the oxide of Group VB to the barrier layer has a positive effect on the coefficient of thermal expansion (CTE) so as to make the barrier layer compatible with the silicon containing substrate, particularly, silicon nitrides. In a preferred embodiment.of the present invention the article can include one or more intermediate layers between the silicon based substrate and the barrier layer. The intermediate layer(s) serve(s) to provide enhanced adherence between the barrier layer and the substrate and/or to prevent reactions between the barrier layer and the substrate. In addition, the layers can serve to provide for transition between layers of different thermal expansion.
Further objects and advantages of the present invention will appear hereinbelow from the following detailed description.